Precipitation of chromium containing phases in aluminide coated nickel-base superalloy single crystals

The precipitation of chromium-containing phases, in both the B2 type β-phase coating matrix (nominally NiAl) and the substrate of high-activity-pack-aluminized single crystals of a nickel-base superalloy, is considered in this paper. An ‘edge-on’ transmission electron microscopy (TEM) technique is employed to examine the precipitation of M₂₃X₆, σ, α-Cr and other phases after coating and diffusion treatment and subsequent post-coating treatment at 850 and 950 °C. Initial precipitation is dominated by the formation of M₂₃X₆ in both the coating and substrate, however, in the case of single-crystal substrates the formation of this carbon-rich phase is not sustained. M₂₃X₆ precipitation is superceded by the formation of coherent precipitates of the α-Cr phase which effectively retains the basis but removes the superlattice of the β-matrix. Extensive precipitation of α-Cr has the effect of changing the balance of chromium to molybdenum in solution in the β-phase and further precipitation is dominated by Σ-phase intermetallics and other Cr-Mo-containing phases.